Traditionally, digitally controlled printing capability is accomplished by one of two technologies. The first technology, commonly referred to as continuous stream or continuous inkjet printing, uses a pressurized ink source which produces a continuous stream of ink drops. The ink drops are directed to an appropriate location using one of several methods (electrostatic deflection, heat deflection, gas deflection, etc.). When no print is desired, the ink drops are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink drops are not deflected and allowed to strike a print media. Alternatively, deflected ink drops can be allowed to strike the print media, while non-deflected ink drops are collected in the ink capturing mechanism.
The second technology, commonly referred to as drop on demand inkjet printing, provides ink drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink drop through a nozzle bore that strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle bore, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
Conventional drop on demand inkjet printers utilize a pressurization actuator to produce the inkjet drop at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink drop to be expelled. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate. With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink drop to be expelled.
It is known to print directly to hydrophobic media, such as vinyl, etc., using solvent-based dye inks ejected from piezoelectric inkjet printheads. However, due to the high cost of piezoelectric printer systems (ranging from $20,000 to $100,000) these printers are too costly for small volume printing jobs and some industry operating environments, such as small shop environments, etc. Piezoelectric printer systems typically require an ink (often solvent based) having a high viscosity which are not suitable for use with thermal inkjet printing systems. Additionally, the cost associated with piezoelectric printer systems can be contrasted with the cost associated with thermal inkjet printing systems which typically range from $10,000 to $15,000.
In order to solve this industry problem, an affordable inkjet printer that prints directly on untreated and uncoated hydrophobic media, such as vinyl, etc., and still offers a competitive cost per print, is desired in the industry. Additionally, pigmented inks, as compared to dye based inks, have enhanced properties such as colorfastness, lightfastness, image durability, etc., when exposed to extreme weather conditions, for example, those present in an outdoor environment. However, conventional aqueous (water) based pigmented inks having low viscosity, a characteristic that makes these inks suitable for ejection from thermal inkjet printheads, have difficulty wetting and adhering directly to non-porous, uncoated hydrophobic media. This is because an ink drop can be heated to a temperature approaching of 300° C. (for a brief time period) in a thermal inkjet printing system which typically leads to kogation of the ink and printhead nozzle malfunction. As such, the industry is challenged to produce inkjet ink compositions having improved image quality characteristics that can adhere to non-porous, untreated, uncoated hydrophobic media and thermal inkjet printing systems capable of delivering these inks to the media.
U.S. Pat. No. 5,734,392, issued to Cornell on Mar. 31, 1998, maintains operating temperatures by heating a thermal inkjet printhead. A silicon chip has embedded resistors (commonly referred to as substrate heaters) positioned at opposite ends of two rows of nozzle bores. Each nozzle bore has an associated drop forming resistor which vaporizes a portion of the liquid ink under the nozzle bore causing an ink drop to be ejected from the nozzle bore. Typically, conventional water based pigmented inks are heated by this printhead to a temperature not exceeding 50° C. in order to create a favorable operating environment without causing the inks to overheat. Overheating of the inks creates a larger than desired ejected ink drop size which, in turn, can create undesired image artifacts on a recording media reducing the quality of the printed image.
U.S. Pat. No. 6,382,759 B2, issued to Maeda et al. on May. 7, 2002, discloses an inkjet recording apparatus having a heater that heats a recording material to a predetermined temperature range at an ink recording position. The apparatus also includes a printhead that ejects ink to the recording material when the recording material is located at the recording position. The ink ejected by the printhead contains a substance that thickens when exposed to heat. The apparatus also includes a measuring means for monitoring the duration of time the printhead is located at the recording position. Additionally, a means for controlling the duration that the printhead remains at the recording position is included in the apparatus in order to prevent the nozzles and/or other portions of the printhead from being adversely affected by the heat. The ink used with this apparatus has a reversible temperature viscosity relationship. That is, the viscosity of the ink increases when the ink is exposed to heated conditions which causes the ink to gel and remain on the surface of the recording material. However, if the ink contained inside the printhead is allowed to increase in temperature, that ink will also gel and clog the nozzle of the printhead. Accordingly, the printhead must be repositioned away from the recording position periodically and allowed to cool to an acceptable temperature.
U.S. Pat. No. 4,970,528, issued to Beaufort et al. on Nov. 13, 1990, discloses a paper handling and ink drying apparatus which is part of a page width inkjet printer. An omnidirectional source of heat is positioned adjacent the paper exit path of an inkjet printer capable of radiating heat about an 180 degree contoured area relative to the location of the heat source. Paper from the inkjet printer is passed over this 180 degree contoured area as it exits the inkjet printer and moves toward a paper receiving and stacking area. The movement of the paper over the 180 degree contoured area is achieved by providing a semi-cylindrical contoured heat reflector which is an integral part of the printer apparatus.
Aqueous based ink compositions containing polymeric binders, for example those disclosed in U.S. Pat. No. 5,133,803, issued to Moffatt on Jul. 28, 1992, U.S. Pat. No. 5,364,462, issued to Crystal et al. on Nov. 15, 1994, and U.S. Pat. No. 6,239,193, issued to Cheng et al. on May. 29, 2001, are also known in the art. However, the binders added to these ink compositions are in very low concentration because adding a high concentration of binder increases the viscosity of the ink composition causing ink kogation and nozzle clogging which creates ink ejection problems when the inks are ejected through thermal inkjet heads.
As such, there is a need for a thermal inkjet printing apparatus using aqueous based pigmented inks capable of wetting and adhering directly to non-porous, uncoated, untreated hydrophobic media having improved image quality characteristics.